Terminal screw for electrical connectors



Feb. 25, 1964 H. J. MODREY TERMINAL SCREW FOR ELECTRICAL CONNECTORS Filed NOV. 22, 1961 Prior Ar/ HENRY J. MODREY United States Patent Q 3,122,496 TERlt HNAL SCREW FGR ELEtjTRl'QAL CONNECTGRS Henry J. Modrey, Eagle Drive, Stamford, Conn. Filed Nov. 22*, 196i, Ser. No. 154,255 9 Claims. (Cl. 339-95) The present invention relates to binding screws for securing a looped wire to a base, and more particularly to screws for electrical terminals in which the underside of the screw head presses upon a looped wire conductor when the screw is tightened.

Screw terminals of the type referred to, are universally used in electrical connections for both solid and stranded wire conductors, usually made of copper or aluminum, and their electrical characteristics are well understood. Tightening down the screw on the wire loop puts the wire under compression and tends to flatten it. The elastic energy which establishes contact pressure, is stored by compressing the Wire and by elongating the screw shank. In a No. 4 (0.112") binding screw, the force exerted in clamping a No. 24 gauge (0.020") copper wire is about 135 lbs. This represents about 2.7 mil pounds of elastic energy, stored approximately one-half in the compressed wire and one-half in the elongated screw. A force of such magnitude tends to damage the wire.

Furthermore, the energy storage is not stable and constant. Due to the cold working to which the wire conductor is subjected when being tightened, the wire conductor builds up hardness which reduces its tendency to recover its original shape. The wire remaining under strong compression, rapidly develops cold flow and will collapse gradually with a corresponding decrease of its elastic energy. Even if it were feasible to re-tighten a terminal screw every few weeks, the process would only repeat itself until the wire is deformed beyond its capability of elastic recovery. A screw terminal as heretofore known is therefore not capable of maintaining the contact pressure which it had when originally installed. This decrease in contact quality is well known and it is in large measure responsible for the failure of electrical equi ment which was carefully checked and tested when assembled and wired but failed to perform satisfactorily after a comparatively short period of storage or use.

Various measures are in use to counteract diminishing contact pressure in screw terminals. By way of example, a springy washer interposed between binding head and wire loop will to a certain extent equalize the pressure exerted by the screw and provide follow-up pressure to compensate for the gradual flattening of the wire. A springy washer has several disadvantages. It is a separate and additional part, it increases the wiring time, and it adds an additional electrical joint to the circuit. Furthermore, the pressures used are quite considerable and tend to cause the collapse of conducting non-ferrous spring washers very quickly.

Another approach is the use of fiat tongue terminations attached to the wire. Since such ring or fork-shaped terminals are usually made from fiat stock, continued pressure on them produces no further appreciable flattening of the terminal surface, and t e electrical joint tends to keep tight. However, such terminals are separate components and must first be attached to the wire by crimping, thus adding another operation and another electrical joint which is always undesirable. It is significant that the constancy of the electrical connection produced by non-collapsible flat ring or fork terminals has brought about their universal acceptance, notwithstanding their cost, the expanse of the crimped attachment, and the additional electrical joint between terminal and conductor.

The progressive collapse of the conductor wire held by 3,lZZ,4% Faterited Feb. 25, 1984 an ordinary terminal screw loosens the screw. Such terminals are therefore subject to dislocation by vibration and shock. They can be secured by various means, such as lock washers or locking thread inserts, or by the use of flat terminations attached to the wire. All these means increase the cost and detract from the overall conductivity of the electrical joint by the interpolation of additional parts.

it is a broad object of the present invention to provide a novel and improved terminal screw which avoids the aforementioned shortcomings without requiring the use of additional parts.

t is also a broad object of the invention to provide a novel and improved terminal screw of the general kind above referred to whi h is better capable of maintaining the constant pressure with which it was installed than other terminal screws as heretofore known.

A more specific obiect of the invention is to provide a novel and improved terminal screw the head of which is divided into two portions, one of which compresses, flattens and positively locks part of the wire loop while the other portion of the head is adapted to maintain contact constant pressure upon the wire loop without appreciably flattening a solid wire or damaging a stranded wire.

Another cbiect of the invention is to provide a novel and improved terminal screw which when tightened is less apt to damage solid or stranded conductors than other terminal screws heretofore known.

Still another object of the invention is to provide a novel and improved terminal screw with a petal-shaped, partially springy head which secures the screw against dislocation under the influence of shock and vibration.

A further object of the invention is to provide a novel and improved terminal screw comprising a sharp-edged secondary contact surface between the head and the threads of the screw. This contact surface establishes additional contact between the conductor and the terminal at right angles to the plane of the principal pressure contact area of the terminal.

A still further object of the invention is to provide a novel and improved terminal screw coacting with a threaded base provided with means to grip the wire so as to wrap the wire more tightly against the sharp edge secondary contact surface of the terminal screw.

Yet another object of the invention is to provide a novel and improved terminal screw containing a relieved portion of screw threads which facilitates the elongation of the screw under tension and thereby assists in the development and storage of elastic energy.

Generally speaking, the invention resides in dividing the binding head of the screw into portions of different elasticity. ()ne of these portions is in the form of a substantially solid and rigid narrow strip including a driving slot and the other portion is springy and elastic.

The divided head, when tightened upon the wire, deforms the wire only below the narrow solid strip. This deformation of a small part of the wire loop is necessary because it locks the wire positively against pull. A fully elastic head would permit the wire to be pulled out. The elastic part of the head gives under pressure and therefore leaves the larger part of the wire-loop substantially in its original configuration. The greater portion of the elastic energy stored in the wired terminal therefore resides in the elastic part of the screw head and not in the wire.

The springiness of the binding head is especially useful for stranded conductors which tend to be torn by the pressure of a standard binding screw.

In one embodiment of the invention, the elastic portion of the screw head is formed with gentle radial undulations. When tightened, the valley portions of the undulations exert stronger pressure upon the conductor wire than the peaks. The coaction of these valleys and peaks with the conductor wire can be compared to a rudimentary ratchet. This arrangement effectively prevents dislocation of the screw head under the influence of vibration. It constitutes in effect an integral lock washer.

In another embodiment of the invention, storage of elastic energy in the screw is facilitated by leaving some of the screw threads free to move slightly relative to the threaded base of the terminal, whereby a considerably longer portion of the screw shank is made available for elongation when the screw is tightened. As a result, overstress is avoided which frequently leads to the rupture of binding screw heads, especially if the terminal was installed by feel and not by means of a torque tool.

In still another embodiment of the invention, a secondary contact surface is disposed at right angles to the prinicpal pressure contact surface. For this purpose, the terminal screw is formed with a contact portion of a sharp cornered cross-section such as a square or starshaped cross section located below its head. When the wire is wrapped looped around the screw under the influence of the turning motion of the head, the corners of the secondary contact surface dig into the wire. This feature is useful because the sharp corners of the secondary contact surface will break through an oxide film which may have formed on the wire.

The undulations of the screw head serve to grip the wire and wrap it tightly around the square contact section, and finally to hold it permanently under circumferential tension. The slight upward slant of the underside of the screw head also coacts in pushing the wire inwards during the tightening process and thus tightening the wrap.

The provision of two separate contact arrangements within one screw terminal at no increase in manufacturing or installation cost, is advantageous for the overall contact quality of the electrical connection. It has special advantages where vibration is a factor because the two contact surfaces are in different planes. Vibration effects are generally most severe in one direction only which leaves one of the two contact surfaces unaffected.

Other and further objects, features and advantages of the invention will be pointed out hereinafter and set forth in the appended claims forming part of the application.

In the accompanying drawing, several preferred embodiments of the invention are shown by way of illustration and not by way of limitation.

In the drawings:

FIG. 1 is a perspective view of a terminal screw according to the invention, with part of the screw head cut away;

FIG. 2 is a sectional view of the terminal screw tightened on a wire, immediately after the tightening of the screw. The section follows the direction of the driving slot in the screw head;

FIG. 3 is a section of the terminal shown in FIG. 2, taken at right angles to that of FIG. 2;

FIG. 4 represents the terminal shown in FIG. 2, after a lapse of time during which the wire has continued to flatten due to pressure and cold flow;

FIG. 5 is the wire loop shown in FIGS. 2 through 4-;

PEG. 6 is a section of a prior art screw terminal, in a configuration corresponding to FIG. 2;

FIG. 7 is a section through a screw terminal of FIG. 6, in a configuration corresponding to FIG. 4;

FIG. 8 is the Wire loop shown in FIGS. 6 and 7;

FIG. 9 is a section of the wire terminal shown in HG. 2, taken along the line 99 through the square secondary contact surface of the terminal screw;

FIG. 10 is a section similar to that of FIG. 9 but modified to show a star-shaped secondary contact surface;

FIG. 11 is a perspective view of a double terminal showing two terminal screws according to the invention, and further showing wire retaining notches in the joint terminal base;

FIG. 12 is a partial section through a terminal screw aaoe according to one embodiment of the invention and a threaded terminal base, the terminal screw being relaxed; and

FIG. 13 is a partial section similar to FIG. 12, but with the terminal screw tightened and elongated.

Referring now to the figures in detail, and more specifically to FIG. 1, the terminal screw as shown in this figure comprises a threaded shank 1t and a binding head 11.

The threads of the shank may either be of uniform diameter along the length of the shank or they may have the specific and useful configuration which will be more fully explained in connection with FIGS. 12 and 13.

The binding head is generally of circular peripheral outline and it is divided in a solid and rigid or unyielding portion 12 and a flexible or yielding portion 13. The rigid portion of the head is in the form of a comparatively narrow strip extending across the diameter of the head and includes a driving slot 12a.

The flexible portion may either have a smooth surface, or it may preferably be slightly undulated, as shown. The inner side of the head, that is, the side facing the shank, is slightly inwardly slanted or dished toward the root of the shank.

The inner surface of the head constitutes the primary contact surface of the terminal screw. A sharp cornered secondary contact surface 14 is preferably interposed between the threaded portion of the shank and the root thereof. This secondary contact surface may have the square cross-section shown in FIGS. 1 and 9, or it may have any other suitable sharp cornered peripheral outline such as the star shape 14 shown in FIG. 10.

FIGS. 2, 3 and 4 show the terminal screw according to FIG. 1, screwed in a base 15 to hold the looped end of a wire conductor 16 in position and in contact with the base.

As is apparent from the previous description and the configuration of the binding head as shown, the binding head exerts a strong pressure upon the wire loop only at the areas which are in contact with the solid strip or rib 12. Accordingly, only the corresponding areas of the wire loop 16:: will be flattened more or less as is indicated in FIG. 5 at 16b. The remaining portions of the wire loop underlie the flexible portions 13 of the binding headand hence, retain their original round configuration. In other words, only a small fraction of the wire loop undergoes a deformation and this favorable condition is not affected when the wire is gradually further flattened due to cold flow, as is indicated in FIG. 4. As is evident, such further flattening cannot spread to the major portion of the wire loop which underlies the elastic portions of the binding head.

As the binding head is tightened, the wire of the loop is pushed inwardly by the dished or slanted configuration of the inner side of the binding head and thus strongly pressed against the sharp corners of the secondary contact surface, as shown in FIGS. 9 and 10. These sharp corners will dig somewhat into the material of the wire thereby breaking through any oxide film which may have formed and generally improving the contact quality.

The specific and useful function of the binding head. i 7

according to the invention can be most readily apprehended by comparing FIGS. 2, 4 and 5 with FIGS. 6, 7 and S, which show a standard binding head 17 of uniform rigidity and having a flat inner contact surface. A binding head of this kind does and must exert upon the wire loop ifia of a wire conductor 18 a strong and uniform pressure along its entire length. Accordingly, the wire loop will be flattened along its entire length as is indicated at 18b and the flattening of the wire loop will be further aggravated due to the unavoidable gradual action of cold flow as is indicated in FIG. 7.

FIG. 11 shows a double sided terminal in which are inserted two terminal screws 21 and 22. The terminal screws are similar in principle to the terminal screw described in connection with FIG. 1, except that the flexible portions 21a and 21b of the binding head, which are shown undulated in FIG. 1, are left smooth. However, it is also possible to use terminal screws such as shown in FIG. 1 in the terminal of FIG. 11. The terminal strip 23 is turned up at both ends at 23a and the edge of each turned up portion is formed with a notch 23b. This notch serves as a restraint for the blank wire end of a conductor 24 whereby a tight wrap around of the wire in FIG. 9 or FIG. 10 is effected when the terminal screws are tightened.

When a terminal screw is tightened in a terminal base 25, the threads of the shank will become tightly wedged in the corresponding threads of the terminal base. Furthermore, the shank of the screw undergoes a certain elongation which, in terminal screws as heretofore known, is substantially limited to the portion of the shank protruding from the base and this portion in turn is determined by the thickness of the wire which is clamped against the base by the binding head. Such concentration of the elongation of the shank in a very short portion thereof tends to overstress the shank portion involved and is the cause for the tendency of the screw head to sanp off when the screw head is tightened. According to the invention, the diameter of the screw threads adjacent to the binding head is somewhat reduced as is shown in FIGS. 12 and 13 for a threaded shank 10a. Such reduction in diameter of these screw threads provides excess clearance near the top side of the base and as a result, the elongation of the screw when the same is tightened is extended over a correspondingly elongated length of the shank. It has been found that a relief of the screw threads near the binding head increases the elasticity of the screw and effectively counteracts the tendency of the head to snap off when or after the screw is tightened.

While the invention has been described in detail with respect to certain now preferred examples and embodiments of the invention, it will be understood by those skilled in the art after understanding the invention, that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended, therefore, to cover all such changes and modifications in the appended claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A terminal screw for electrical wiring terminals comprising a threaded shank, and a substantially disc shaped dished binding head, the hollow side of the disc facing the shank, said disc having on its opposite side a substantially rigid strip shaped portion extending across its diameter and elastic portions completing the disc shape on opposite sides of said strip for clamping a wire wrapped around the shank between said head and a rigid plane base.

2. A terminal screw for electrical wiring terminals comprising a threaded shank, and a substantially disc shaped dished binding head, the hollow side of the disc facing the shank, said disc having on its opposite side a substantially rigid strip shaped portion extending across its diameter and including a driving slot and elastic portions extending from opposite sides of said strip for clamping a wire wrapped around the shank between said disc portion and a plane base, said elastic portions including radially oriented undulations.

3. A terminal screw for electrical wiring terminals comprising a binding head havin a substantially rigid portion extending transversely across the head and a springy portion extending from both sides of the rigid portion, and a shank having a threaded portion and a nonthreaded portion including sharp corners in cross-section and disposed adjacent to said head, the side of the head facing the shank having said spring portion and constituting a first contact surface and the sides of said smooth shank portion constituting a second contact surface for clamping a wire Wrapped about said second contact surface between the head and a rigid base.

4. A terminal screw according to claim 3, wherein said non-threaded shank portion is of square cross-section.

5. A terminal screw according to claim 3, wherein said non-threaded shank portion is of star shaped crosssection.

6. A terminal screw for electrical Wiring terminals comprising a threaded shank and a substantially discshaped binding head, the hollow side of the head facing the shank, said head having a substantially rigid strip shaped portion extending across the diameter of the head and including a driving slot and elastic portions on opposite sides of said strip, said shank including a threaded portion and a non-threaded portion including sharp corners in cross-section and disposed adjacent to the head, the side of the head facing the shank constituting a first contact surface and the sides of the smooth shank portion constituting a second contact surface for clamping a wire wrapped about the second contact surface between the first contact surface and a rigid base.

7. A terminal screw for electrical wiring terminals comprising a threaded shank and a substantially disc shaped binding head, the hollow side of the head facing the shank, said head having a substantially rigid strip shaped portion extending across the diameter of the head and including a driving slot and elastic portions on opposite sides of said strip, sm'd shank including a threaded portion and a non-threaded portion including sharp corners in cross-section and disposed adjacent to the head, the side of the head facing the shank constituting a first contact surface and the sides of the smooth shank portion constituting a second contact surface for clamping a wire wrapped about the second contact surface between the first contact surface and a rigid base, the threads of the threaded shank portion adjacent to the head being slightly reduced in diameter in reference to the remaining threaded shank portion.

References Cited in the file of this patent UNITED STATES PATENTS 1,395,788 Anthony Nov. 1, 1921 2,694,800 Sunderhauf Nov. 16, 1954 2,933,006 Gibb Apr. 19, 1960 3,650,318 Van Der Wissel Aug. 21, 1962 FOREIGN PATENTS 393,544 Great Britain June 8, 1933 495,717 Belgium Sept. 1, 1950 1,079,711 Germany Apr. 14, 1960 

9. A TERMINAL SCREW FOR ELECTRICAL WIRING TERMINALS COMPRISING A THREADED SHANK, AND A SUBSTANTIALLY ROUND BINDING HEAD HAVING ON ITS SIDE OPPOSITE THE SHANK A SUBSTANTIALLY RIGID RIB EXTENDING ACROSS THE DIAMETER OF THE HEAD, THE SIDE OF THE HEAD FACING THE SHANK HAVING ON OPPOSITE SIDES OF THE RIB SPRINGY PORTIONS SLIGHTLY SLANTED TOWARD THE ROOT OF THE SHANK FOR CLAMPING A WIRE WRAPPED AROUND THE SHANK BETWEEN SAID HEAD AND A RIGID PLANE MOUNTING BASE. 